|Rotz, Clarence - Al|
Submitted to: Transactions of the ASABE
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/1/2009
Publication Date: 10/30/2009
Citation: Chaoui, H., Montes, F., Rotz, C.A., Richard, T. 2009. Dissociation and ammonia mass transfer from ammonium solutions and dairy cattle manure. Transactions of the ASABE. 52(5)P1695-1706. Interpretive Summary: Ammonia volatilization from manure in animal agriculture is a growing concern due to its potential effect on the environment and the loss of nitrogen from the farm. Ammonia emitted to the atmosphere can combine with other molecules to form smog or it can precipitate onto lakes and other sensitive ecosystems causing over-fertilization. The lost nitrogen is also a valuable crop nutrient that may be replaced with inorganic fertilizer produced from fossil fuel. Ammonia volatilization varies depending on the temperature, manure pH, air speed, manure handling operations, and other manure characteristics, which makes it difficult and expensive to measure. Another option is to use mathematical models that represent the important physical and chemical processes occurring in the manure. Models have been used to estimate ammonia losses from swine, cattle, and poultry farms. Important parameters in volatilization models are the ammonium dissociation constant, Henry's law constant, and the mass transfer coefficient, and reported values for these parameters vary widely among models. Laboratory experiments were conducted to measure these parameters and evaluate a current volatilization model. We found that the dissociation constant used in most manure models under predicted the dissociation from dairy cattle manure. The mass transfer coefficient commonly used in the same models over predicted volatilization. Thus, combining these relationships in a current model provided reasonable predictions of ammonia volatilization. Dissociation from cattle manure was consistently higher than that from the ammonium solution, which suggests that the emission of carbon dioxide from manure increases the pH of the manure surface and this increases the amount of ammonia emitted. This new understanding of the ammonia volatilization process will improve our ability to model and predict emissions from farms, which will ultimately lead to the development of strategies for reducing emissions and conserving nitrogen on our farms.
Technical Abstract: Process-based models are being used to predict ammonia (NH**3) emissions from manure sources, but their accuracy has not been fully evaluated for cattle manure. Laboratory trials were conducted to measure the dissociation and mass transfer coefficient for NH**3 volatilization from media of buffered ammonium-water solution and dairy cattle manure. Effects of ionic strength, ammoniacal N concentration, temperature and pH of the media, and air velocity over the media were evaluated. As represented in existing models, media type, temperature, and pH were verified to influence ammonium dissociation and NH**3 mass transfer. Model prediction underestimated measured dissociation values by 5% in solution trials and 94% in manure trials. Prediction error for manure was due to an increase in surface pH created through the formation and emission of carbon dioxide (CO**2). Ammonia flux from both the ammonium solution and manure surfaces was affected by temperature and air velocity over the media. An existing model of NH**3 flux predicted emission rates from manure surfaces more accurately than that from ammonium solution with average errors of -16 and +81%, respectively. Temperature had a significant effect on the error between predicted and measured rates with the greatest error at temperatures over 25 deg C. Despite the error in predicting dissociation from manure, a current model adequately represented NH**3 emission from a fresh manure surface where the surface pH did not have time to increase as a result of CO**2 emission. The results imply that model refinement is needed to improve accuracy in predicting NH**3 emissions from dairy cattle manure. An important consideration in further development of process-level models is the measurement and prediction of manure surface pH as affected by CO**2 formation and emission.